This invention generally relates to an apparatus and program for determining linearity of tire characteristic, which determines the condition of tires of a traveling vehicle in view of linearity of the tire characteristic, and more particularly to a vehicular driving operation apparatus, and a steering control method for steer-by-wire (SBW) vehicles, in which thus-determined condition of the tire is utilized.
Steering gears of vehicles are available in a wide range of variations, such as front-wheel steering gears, rear-wheel steering gears, steering gears with a steering shaft and steerable wheels linked mechanically, and SBW steering gears. Among these types of steering gears, a yaw-rate feedback active steering system known in the art may be provided in order to stabilize vehicle""s behavior. The yaw-rate feedback active steering system is designed to compare an actual (actually measured) yaw rate (as an indicator of actual or measured vehicle behavior) with a standard or normative yaw rate (as an indicator of standard or normative vehicle behavior) whereby the directional control for a vehicle is exercised so as to make the actual yaw rate coincide with the standard yaw rate. The standard yaw rate is calculated from a steering angle of the steering shaft, a steering angle (rack position) of the steerable wheels, or other factors, whereas the actual yaw rate is measured with a yaw rate sensor or other vehicle behavior detecting means. The use of this type of active steering system allows the steering gear to provide a stabilized yaw rate in steering (or aiding a driver in steering) the vehicle, and thus serves to effectively stabilize the vehicle.
Utility of the active steering system as described above holds true of cases where the standard yaw rate is greater than the actual yaw rate. However, an adverse effect would be brought about in cases where the vehicle tends to undergo understeer and where the vehicle is on a snowy road or the like, with the actual yaw rate thereof measuring a small value. To be more specific, without reflecting a slipperiness of the road surface, non-linear model of tire characteristic or the like on the control of the steering gear, the steering gear would disadvantageously attempt to make the actual yaw rate closer to the standard yaw rate and to steer the vehicle more so as to increase the steering angle, thus inducing further understeer.
Determination of the slipperiness of road surfaces requires an expensive sensor (e.g., sensors for which the fifth wheel need be installed). The non-linear model of tire characteristic makes the control process complicate. In view of these disadvantages, another approach may be deemed applicable in which the active steering system is disabled in understeer conditions (i.e., standard yaw rate greater than actual yaw rate); however, this approach would spoil the advantages of the active steering system in cases where the vehicle usually undergoes understeer, with the result that equipment of the active steering system would become ineffectual.
Therefore, it is one exemplary and general object of the present invention to provide an apparatus and program for determining linearity of tire characteristic, in which the condition of tires of a traveling vehicle can be determined using a commonly available sensor and with comparative ease. Another exemplified and more specific object of the present invention is to provide a vehicular driving operation apparatus and steering control method for SBW vehicles, in which the control for a vehicle is exercised utilizing thus-determined condition of the tires.
The inventors and their colleagues, who have diligently carried on a research to address the above-described problems, focused on the point that comparison between standard vehicle behavior (as indicated by a standard or normative yaw rate) and actual vehicle behavior (as indicated by an actual yaw rate) can be used to determine whether tires stand in a non-linear region or linear region of tire characteristic, and finally completed the present invention.
[Apparatus for Determining Linearity of Tire Characteristic]
According to one exemplified aspect of the present invention, there is provided an apparatus for determining linearity of tire characteristic comprising: a standard vehicle behavior arithmetic unit that calculates a standard vehicle behavior indicator based upon at least one of an operation amount of an operation unit with which a driver of a traveling vehicle performs a steering operation and an actually steered amount of steerable wheels of the traveling vehicle; and a tire characteristic linearity determination unit that compares an actual vehicle behavior indicator with the standard vehicle behavior indicator, to determine whether tires of the traveling vehicle stand in a linear region or in a non-linear region of the tire characteristic. The operation amount of the operation unit, actually steered amount of the steerable wheels, and actual vehicle behavior indicator may be detected by an operation amount detector, a steered amount detector and a vehicle behavior detector, respectively.
In this arrangement, a standard vehicle behavior indicator is calculated based upon detected values transmitted from at least one of the operation amount detector and the steered amount detector, and the resulting standard vehicle behavior indicator is compared with the actual vehicle behavior indicator detected by the vehicle behavior detector, so that determination is made whether the tires stand in the linear region or in the non-linear region. The vehicle behavior indicator includes, but not limited to, a yaw rate as described in one exemplary embodiment which will be described later; for example, a lateral acceleration may be utilized instead. Sensors required for this arrangement, as denoted above by the operation amount detector and/or the steered amount detector, and the vehicle behavior detector, may be selected among general purpose sensors that are inexpensive and thus readily available. Alternatively, sensors with which most vehicles have already been equipped may be used as all or part of the above detectors.
In the non-linear region of tire characteristic, the vehicle is placed in a condition to become unsteerable or undergo understeer even if the steering angle (steering amount) is increased. In contrast, in the linear region of the tire characteristic, the vehicle is in a condition to be steered in an intended direction as the steering angle is increased.
The above tire characteristic linearity determination unit may include: a vehicle behavior deviation change rate arithmetic part that calculates a vehicle behavior deviation by subtracting the actual vehicle behavior indicator from the standard vehicle behavior indicator, and calculates a rate of change of the vehicle behavior deviation; a sign determination part that determines a first sign of one of the vehicle behavior deviation and the rate of change of the vehicle behavior deviation, and a second sign of the actual vehicle behavior indicator; a tire characteristic linearity determination part that determines linearity of tire characteristic, wherein if the first and second signs are both positive and the rate of change of the vehicle behavior deviation is greater than a first prescribed value, and if the first and second signs are both negative and the rate of change of the vehicle behavior deviation is less than a second prescribed value, it is determined that the tires of the traveling vehicle stand in the non-linear region of the tire characteristic.
If the first sign (positive (+)/negative (xe2x88x92)) of the rate of change of the vehicle behavior deviation and the second sign (positive (+)/negative (xe2x88x92)) of the actual vehicle behavior indicator are both positive (+), it is determined that the tires of the traveling vehicle stand in the non-linear region of the tire characteristic when the rate of change of the vehicle behavior deviation is greater than a first prescribed value. On the contrary, the first and second signs are both negative (xe2x88x92), it is determined that the tires of the traveling vehicle stand in the non-linear region of the tire characteristic when the rate of change of the vehicle behavior deviation is less than a second prescribed value. In other words, if the above conditions are satisfied, it is determined that the actual vehicle behavior indicator is not increased irrespective of the driver""s intention to steer the vehicle, or it is determined that the actual vehicle behavior does not sufficiently follow the standard vehicle behavior (the actual vehicle behavior indicator is not increased) even though the steerable wheels are actually steered; therefore, it is determined that the tires stand in the non-linear region of the tire characteristic.
[Program for Determining Linearity of Tire Characteristic]
A program for determining linearity of tire characteristic as another exemplary aspect of the present invention characteristically causes a computer to execute a process as follows to determine whether tires of a traveling vehicle stand in a linear region or in a non-linear region of the tire characteristic. The process includes the steps of: (1) receiving at least one of an operation amount of an operation unit with which a driver of the traveling vehicle performs a steering operation and an actually steered amount of steerable wheels of the traveling vehicle, which are detected by an operation amount detector and a steered amount detector, respectively; (2) calculating a standard vehicle behavior indicator based upon the at least one of the operation amount of the operation unit and the actually steered amount of the steerable wheels; (3) receiving an actual vehicle behavior indicator detected by a vehicle behavior detector; (4) calculating a vehicle behavior deviation by subtracting the actual vehicle behavior indicator from the standard vehicle behavior indicator; (5) calculating a rate of change of the vehicle behavior deviation; (6) determining a first sign of one of the vehicle behavior deviation and the rate of change of the vehicle behavior deviation, and a second sign of the actual vehicle behavior indicator; (7) if the first and second signs are both positive and the rate of change of the vehicle behavior deviation is greater than a first prescribed value, determining that the tires of the traveling vehicle stand in the non-linear region of the tire characteristic; and (8) if the first and second signs are both negative and the rate of change of the vehicle behavior deviation is less than a second prescribed value, determining that the tires of the traveling vehicle stand in the non-linear region of the tire characteristic.
A processor or CPU (Central Processing Unit) in a computer loads a program for determining linearity of tire characteristic stored in a storage device into memory or RAM (Random Access Memory), and executes the process steps as described above to implement functionality of each step on the computer. To be more specific, the program performs calculation of a standard vehicle behavior indicator from detected values received from at least one of the operation amount detector and the steered amount detector, and comparison between the detected values and an actual vehicle behavior indicator detected by the vehicle behavior detector (determination of signs of the detected values), to determine whether the tires of the traveling vehicle stand in the linear region or in the non-linear region of the tire characteristic. The program may be duplicated, and each stored in a medium and placed in physical distribution channels, or transmitted via communications network.
[Vehicular Driving Operation Apparatus]
A vehicular driving operation apparatus as yet another exemplary aspect of the present invention includes: an operation unit with which a driver of a traveling vehicle performs a steering operation; a steering amount detection unit including at least one of an operation amount detector that detects an operation amount of the operation unit and a steered amount detector that detects an actually steered amount of steerable wheels of the traveling vehicle; a steering motor that drives the steerable wheels to be steered; a control unit that controls the steering motor based upon at least one of the operation amount of the operation unit and the actually steered amount of the steerable wheels; a vehicle behavior detector that detects an actual vehicle behavior indicator; the apparatus for determining linearity of tire characteristic as described above; and a steering amount reduction control unit that controls the steering motor so that the steering motor drives the steerable wheels to be steered back if the tire characteristic linearity determination unit determines that the tires of the traveling vehicle stand in the non-linear region of the tire characteristic.
With this vehicular driving operation apparatus, if the tires stand in the non-linear region of the tire characteristic, the steering motor drives the steerable wheels to be steered back. At this stage, the steering motor may be controlled so that the steering amount is reduced, or the steering motor may be controlled so as to prevent the steering operation of the driver from further steering the vehicle (i.e., to prevent the operation amount of the operation unit from being increased) without actually reducing the steering amount. The steering motor may be an assist motor or the like for use with an electric power steering system which assists a driver in performing a manual steering operation.
One variation of the vehicular driving operation apparatus according to the present invention includes: an operation unit with which a driver of a traveling vehicle performs a steering operation; a reaction force motor that gives a reaction force against operation of the operation unit; a steering amount detection unit including at least one of an operation amount detector that detects an operation amount of the operation unit and a steered amount detector that detects an actually steered amount of steerable wheels of the traveling vehicle; a steering motor that drives the steerable wheels to be steered; a control unit that controls the steering motor based upon at least one of the operation amount of the operation unit and the actually steered amount of the steerable wheels; a vehicle behavior detector that detects an actual vehicle behavior indicator; an apparatus for determining linearity of tire characteristic as described above; and a steering amount reduction control unit that controls the reaction force motor so that the reaction force motor drives the operation unit to move back toward a neutral position if the tire characteristic linearity determination unit determines that the tires of the traveling vehicle stand in the non-linear region of the tire characteristic.
With this vehicular driving operation apparatus, if the tires stand in the non-linear region of the tire characteristic, the reaction force motor drives the operation unit to move back toward a neutral position. At this stage, the reaction force motor may be controlled so that the steering amount is reduced, or the reaction force motor may be controlled so as to prevent the steering operation of the driver from further steering the vehicle (i.e., to prevent the operation amount of the operation unit from being increased). When the reaction force motor has already been driving the operation unit to move back toward a neutral position before the condition of the tires comes into the non-linear region, the reaction force motor may be controlled so that the reaction force toward the neutral position becomes large.
[Steering Control Method for a Steer-by-Wire Vehicle]
A steering control method for a steer-by-wire vehicle as yet another exemplary aspect of the present invention has the following features. The steer-by-wire (SBW) vehicle has a steer-by-wire (SBW) system which includes an operation unit with which a driver performs a steering operation, a steering motor that drives steerable wheels to be steered, and a reaction force motor that gives a reaction force against operation of the operation unit, to perform a steering operation through electric connections. The steering control method for such an SBW vehicle includes: (1) calculating a standard vehicle behavior indicator based upon at least one of an operation amount of the operation unit and an actually steered amount of the steerable wheels; (2) comparing an actual vehicle behavior indicator obtained by detection with the standard vehicle behavior indicator, to determine whether tires of the vehicle during traveling stand in a linear region or in a non-linear region of tire characteristic; and (3) if it is determined that the tires of the traveling vehicle stand in the non-linear region of the tire characteristic, controlling at least one of the reaction force motor and the steering motor to steer the steerable wheels toward such a direction as to reduce the steering amount thereof.
With this method, if it is determined that the tires stand in the non-linear region of the tire characteristic, the reaction force motor and/or the steering motor are controlled to steer the steerable wheels toward the direction to reduce the steering amount of the steerable wheels. The direction to reduce the steering amount is that which makes the steering amount of the steerable wheels reduced and in which the steerable wheels are steered back toward a neutral position; however, whether or not the steering amount should be actually reduced to make the steerable wheels back to the neutral position can be specified as deemed appropriate.
Other objects and further features of the present invention will become readily apparent from the following description of preferred embodiments with reference to accompanying drawings.